In the field of electronics, conductive and/or insulating features are formed on a substrate through photo-lithographic techniques. Essentially, an optical image that represents one or more patterns to be formed onto the substrate is directed onto a layer of photo resist that has been coated onto the substrate. A projection camera projects the optical image onto the photo resist layer from light that has been patterned in accordance with a mask.
In general, a primary measure of an electronic device's sophistication is its smallest feature size. The smallest feature size of an electronic device is largely determined by the sophistication of the lithography techniques and/or equipment employed in the device's manufacture. In particular, the shorter the wavelength of the light that is processed by the photo-lithographic equipment's projection camera optics, the smaller the smallest achievable feature size becomes.
Thus, in general, the smaller the wavelength of the light that is processed by the projection camera's optics, the more sophisticated the projection camera is deemed to be. Presently, considerable work is being done in the development of photo-lithographic equipment that processes light in the Extreme Ultra Violet (EUV) spectra (a range approximately from 10 to 14 nm). Part of the challenge in designing EUV photo-lithographic equipment is designing that portion of the equipment that “pre-conditions” the EUV light prior to illuminating the mask and the entrance pupil of the projection camera.
FIG. 1 shows a simplistic depiction of the cross section of the “shape” of light as it is reflected from the mask at a “ring field” projection camera. According to the depiction of FIG. 1, the light travels substantially along the z axis through arc 101. According to one EUV approach, the arc 101 of the EUV light has a radius R between 116 mm and 124 mm over an angle θ of approximately 30°. Moreover, at least for EUV light, the illumination of the light over the arc 101 is supposed to be highly uniform (e,g., on the order of only 1% variation across the arc 101).
A condenser is used to form light into the appropriate shape and uniformity at the projection camera entry pupil. The condenser can usually be viewed as containing two components: 1) a collector; and, 2) an illumination system. The collector is designed to collect photons from a light source. The illumination system crafts the light from the collector into the appropriate shape for illuminating the mask (arc field) and illuminating the entrance pupil of the projection camera.
An exemplary condenser originally described in U.S. Pat. No. 6,195,201 B1 (hereinafter, “Koch et. al.”) is shown in FIG. 2. The collector 201 includes a light source 203 and a collection mirror 204. The collection mirror 204 directs the light it collects into the illumination system 202. The illumination system 202 includes a pair of faceted mirrors 205, 206. The faceted mirrors 205, 206 effectively break down the light from the collector 201 into a plurality of beams that are recombined by relaying mirrors 207, 208 so as to form light of the proper shape and uniformity at the mask plane 209 of the projection camera.
A problem with EUV condensers is their expense. The cost of an EUV condenser is largely a function of the amount of photon energy that its light source emits. That is, the more photon energy that a light source emits, the more expensive the condenser.